Rechargeable (secondary) lithium-ion batteries are widely utilized in consumer electronic devices such as cell phones and laptop computers owing, in part, to their high energy density. Rechargeable lithium-ion batteries are also useful in power-intensive applications, such as in electric vehicles and power tools. Additional uses for rechargeable lithium-ion batteries, such as in energy grid storage, are possible.
Although rechargeable batteries with other alkali-metal ions, such as sodium and potassium, are less widespread, they may be used in many of the same applications as lithium-ion batteries.
A rechargeable battery stores electrical energy as chemical energy in two electrodes, an anode and a cathode, that are separated from one another inside the battery by an electrolyte and, if the electrolyte is a liquid, by a separator. The separator may be chemically inert on contact with the electrodes, and the separator is permeable to the liquid electrolyte. A chemical reaction that occurs between the two electrodes has two components, an ionic component and an electronic component. The electrolyte is an ionic conductor, but an electronic insulator. Therefore the ionic component of the chemical reaction flows inside the battery as an ionic current in the electrolyte, while the electronic component flows outside the battery in an external circuit as an electronic current. The electronic current may be stopped by opening the electronic circuit and the ionic current may be stopped at open-circuit by an internal electrical field created by a positive charge on the cathode and a negative charge on the anode.
When the battery is discharged, positively charged ions flow inside the battery and the negatively charged electrons flow outside the battery from the anode to the cathode where they recombine to complete the chemical reaction between the electrodes. On discharge, the battery delivers to the external electronic circuit an electric current I at a voltage V for a time Δt until the chemical reaction between the electrodes is completed; thus, the battery transforms the stored chemical energy of the electrode into electric power P=IV. When a rechargeable battery cell is charged, an outside power source supplies a charging current at a charging voltage that reverses the ionic and electronic current flows and, therefore, reverses the chemical reaction to store the applied electric power as chemical energy.
On charge, plating of an alkali metal as the anode from a liquid electrolyte is not smooth; dendrites form and grow from the alkali-metal surface. The dendrites may grow across the electrolyte to the cathode and may create an internal electronic short-circuit that can heat the battery cell and, with a flammable electrolyte, may create a fire. Therefore, in today's lithium-ion battery cells, the anode is commonly carbon, which stores the Li+ ions at a voltage close to that created by metallic lithium. However, if the rate of charge is too high, the charging voltage may become high enough to plate metallic lithium on the carbon and may result in the formation of dendrites.
An additional anode problem may arise because, even if the anode stores Li+ ions at a lower voltage than carbon, the anode may react chemically with the electrolyte unless a solid-electrolyte interphase (SEI) layer forms to stop the anode-electrolyte reaction. The passivating SEI layer on the anode is permeable to the Li− ions, and the Li+ of the SEI are taken from the cathode on the initial charge, which reduces the amount of stored electrical energy.
In order to safely increase the rate of charge, other known means of storing Li+ ions in the anode may include using other insertion hosts than carbon, alloys, and conversion reactions. However, such means of storing Li+ in the anode may still remain unsatisfactory in terms of the density of energy stored.
Thus, present-day rechargeable batteries are unable to incorporate safely an alkali-metal anode because of potential dendrite formation, among other undesirable effects.